When a customer purchases a certificate from the WHMCS marketplace, after it's been issued it'll be automatically installed into their Plesk hosting plan. The installed certificate is automatically named after the domain-name. For example, if the hosting plan is for "mydomain.co.uk", then the installed certificate will also be called "mydomain.co.uk".
When it comes to the certificate renewal, the process completes in exactly the same way - The new certificate is created in Plesk with the same name again. Eg, "mydomain.co.uk".
This means when the customer goes to their "Hosting Settings" page to update the the chosen certificate, they now have 2 "mydomain.co.uk" certificates to choose from. On the next renewal, the same happens again. This duplication causes confusion over time, when a customer can't see from the drop-down list which is the "correct" latest certificate to choose without trial-and-error selecting...
Please consider adding an identifier to the installed certificate name, such as appending a issue or expiry date, so customers know the differences between the installed certificates.